Notwithstanding your good points about it not being fool-proof, it is an important point that your email software doesn't essentially promote or enable corruption of the meta data in the way you describe.

As a business person, nobody would [buy/use] exchange if it was not reasonably secure from an audit trail perspective. The intergrity of the communications is required for many business's who have record retention policy and what not.

Think about an analogy for a bank's accounting system that allowed audit trails to be compromised. Its a huge problem for the purchasing people and the managerial layer that has to sign off on sarbox etc.